One of the most common systems for obtaining an indirect measurement of a patient's blood pressure is a system which measures pressure variations in a pressurized cuff placed on a patient's arm. Such measurements can be made during either the pressurization or the depressurization of the cuff. The cuff pressurization cycle is commonly referred to as the "upramp," while the depressurization cycle is referred to as the "downramp."
Manual measurements of blood pressure are usually taken on the downramp of the cuff pressurization cycle. In this procedure, an occlusion cuff is rapidly inflated to a pressure level above that at which systolic pressure is expected to occur. Once the maximum pressure has been reached, the pressure in the cuff is slowly decreased while the operator listens for changes in the sound characteristics of the pulse, which changes can be correlated with systolic and diastolic pressure.
Although the cuff inflation system described above can be used to obtain accurate indications of blood pressure, it has a number of drawbacks. In particular, the manual inflation system requires that the cuff be inflated significantly above the expected systolic pressure of the patient. Since the patient's systolic pressure is not known in advance, the cuff is often inflated to an unnecessarily high level which can cause discomfort or even tissue damage to the patient. In addition, the uncertainty over the necessary level of inflation often results in the need to reinflate the cuff several times to obtain an accurate reading of blood pressure.
Automatic blood pressure monitoring systems can be adapted to make blood pressure measurements during either the upramp or the downramp portion of the cuff pressurization curve. It is extremely desirable to have a pressurization or depressurization curve when using an automatic monitoring system. Linearity of the curve significantly decreases the length of time required for the measurement and allows the system to make a more accurate measurement. For systems making measurements on the upramp portion of the cuff pressurization curve, it is also desirable to have the cuff quickly pressurized to a predetermined level, e.g. 40 mm of mercury, to avoid taking unnecessary measurements at pressure levels well below those at which systolic and diastolic pressure are expected to occur.